What operation must be performed on HNSW indexes after a database restart?

When dealing with HNSW (Hierarchical Navigable Small World) indexes in the context of databases, it is important to understand how they function and maintain integrity over time. After a database restart, HNSW indexes require rebuilding due to the way their structures and connections are established.

The nature of HNSW indexes relies heavily on their constructed graph to facilitate efficient nearest neighbor searches. This graph must be carefully managed and consistently up to date; otherwise, the index might not perform optimally or may even yield incorrect results after a restart. Rebuilding the index effectively reconstructs these necessary connections and optimizes the search paths based on the latest data.

In contrast, modifying the index directly, assuming it remains intact, or duplicating it after a restart wouldn’t ensure the optimal performance and correctness needed for efficient search operations. Thus, the conclusion is that rebuilding the index is essential for maintaining the integrity and functionality of HNSW indexes post-restart.